Preheat the oven. Preheat your oven to 350°F (175°C). Then, grease an 8.5 x 4.5-inch loaf pan with butter or non-stick spray.
Combine the dry ingredients. Whisk the flour, baking powder, and salt in a large mixing bowl until well combined.
Mix the wet ingredients. In another bowl, beat the sugar, oil (or melted butter), and eggs until smooth.
Combine wet and dry mixtures. Gradually add the dry ingredients to the wet mixture, alternating with the milk. Mix until just combined. Then, stir in the sourdough starter until fully incorporated.
Fold in the blueberries and lemon. Gently fold in the blueberries, lemon zest, and lemon juice (if using). Be careful not to overmix, as this can make the bread tough.
Bake the bread. Pour the batter into the prepared loaf pan. Smooth the top with a spatula. Bake for 50–60 minutes, or until a toothpick inserted into the center comes out with just a few moist crumbs (but no wet batter). Rotate the pan halfway through baking for even cooking. Let the bread cool in the pan for 5 minutes before transferring it to a wire rack to cool completely.
Prepare the lemon glaze. While the bread cools, whisk together the powdered sugar, lemon juice, lemon zest, and milk in a small bowl. The glaze should be thick but pourable. If it’s too thick, add more milk; if it’s too thin, add more powdered sugar.
Drizzle the glaze. Once the bread is fully cooled, drizzle the lemon glaze over the top. Allow the glaze to set for 10–15 minutes before slicing and serving.